Critical factors affecting laccase-mediated biobleaching of pulp in paper industry

Synthesis and application of a phenazine class substrate for high-throughput screening of laccase activity

Biocatalysis is one of the greatest tools for implementing the 12 principles of Green chemistry. Biocatalysts are bio-based, highly efficient and selective, operate at moderate conditions, and can be reused multiple times. However, the wider application of biocatalysts is plagued by a plethora of drawbacks, such as poor stability at operating conditions, inadequate efficiency of catalytic systems, a small number of commercially available biocatalysts, and a lack of substrates or methods for their discovery and development. In this work, we address the lack of suitable substrates for high-throughput screening of laccase by synthesising and investigating a newly developed phenazine-type substrate - Ferbamine. Investigation of Ferbamine pH and thermal stability indicated that its long-term stability in an aqueous medium is superior to that of commercially available substrates and does not require organic solvents. Ferbamine displayed convincing performance in detecting laccase activity on Ferbamine-agar plates in commercial laccase products and the collection of extracts from wild terrestrial fungi (42 species, 65 extracts), of which 26 species have not been described to have laccase activity prior to this work. Incubation of microorganisms on Ferbamine-agar plates showed its compatibility with live colonies. Ferbamine proved to be an easy-to-use substrate, which could be a great addition to the toolbox of methods for the functional analysis of laccases.

Pyrite-assisted degradation of methoxychlor by laccase immobilized on Fe3S4/earthworm-like mesoporous SiO2

Laccase immobilized and cross-linked on Fe3S4/earthworm-like mesoporous SiO2 (Fe3S4/EW-mSiO2) was used to degrade methoxychlor (MXC) in aqueous environments. The effects of various parameters on the degradation of MXC were determined using free and immobilized laccase. Immobilization improved the thermal stability and reuse of laccase significantly. Under the conditions of pH 4.5, temperature 40 °C, and reaction time 8 h, the degradation rate of MXC by immobilized laccase reached a maximum value of 40.99% and remained at 1/3 of the original after six cycles. The excellent degradation performance of Fe3S4/EW-mSiO2 was attributable to the pyrite (FeS2) impurity in Fe3S4, which could act as an electron donor in reductive dehalogenation. Sulfide groups and Fe2+ reduced the activation energy of the system resulting in pyrite-assisted degradation of MXC. The degradation mechanism of MXC in aqueous environments by laccase immobilized on Fe3S4/EW-mSiO2 was determined via mass spectroscopy of the degradation products. This study is a new attempt to use pyrite to support immobilized laccase degradation.

Proteomic investigation reveals the role of bacterial laccase from Bacillus pumilus in oxidative stress defense

The wide distribution of laccases in nature makes them involved in different biological processes. However, little information is known about how laccase participates in the defense machinery of bacteria against oxidative stress. The present study aimed to elucidate the oxidative stress response mechanism of Bacillus pumilus ZB1 and the functional role of bacterial laccase in stress defense. The oxidative stress caused by methyl methanesulfonate (MMS) significantly induced laccase activity and its transcript level. The morphological analysis revealed that the defense of B. pumilus ZB1 against oxidative stress was activated. Based on the proteomic study, 114 differentially expressed proteins (DEPs) were up-regulated and 79 DEPs were down-regulated. In COG analysis, 66.40% DEPs were classified into the category "Metabolism". We confirmed that laccase was up-regulated in response to MMS stress and its functional annotation was related to "Secondary metabolites biosynthesis, transport and catabolism". Based on protein-protein interaction prediction, two up-regulated DEPs (YcnJ and GabP) showed interaction with laccase and contributed to the formation of laccase stability and adaptability. The overexpressed laccase might improve the antioxidative property of B. pumilus ZB1. These findings provide an insight and the guidelines for better exploitation of bioremediation using bacterial laccase. SIGNIFICANCE: Bacillus pumilus is a gram-positive bacterium that has the potential for many applications, such as bioremediation. The expression of bacterial laccase is significantly influenced by oxidative stress, while the underlying mechanism of laccase overexpression in bacteria has not been fully studied. Elucidation of the biological process may benefit the bioremediation using bacteria in the future. In this study, the differentially expressed proteins were analyzed using a TMT-labeling proteomic approach when B. pumilus was treated with methyl methanesulfonate (MMS). Reactive oxygen species induced by MMS activated the secondary metabolites biosynthesis, transport, and catabolism in B. pumilus, including laccase overexpression. Moreover, the simultaneously up-regulated YcnJ and GabP may benefit the synthesis and the stability of laccase, then improve the antioxidative property of B. pumilus against environmental stress. Our findings advance the understanding of the adaptive mechanism of B. pumilus to environmental conditions.

Recent advances in laccase activity assays: A crucial challenge for applications on complex substrates

Despite being one of the first enzymes discovered in 1883, the determination of laccase activity remains a scientific challenge, and a barrier to the full use of laccase as a biocatalyst. Indeed, laccase, an oxidase of the blue multi-copper oxidases family, has a wide range of substrates including substituted phenols, aromatic amines and lignin-related compounds. Its one-electron mechanism requires only oxygen and releases water as a reaction product. These characteristics make laccase a biocatalyst of interest in many fields of applications including pulp and paper industry, biorefineries, food, textile, and pharmaceutical industries. But to fully envisage the use of laccase at an industrial scale, its activity must be reliably quantifiable on complex substrates and in complex matrices. This review aims to describe current and emerging methods for laccase activity assays and place them in the context of a potential industrial use of the enzyme.

Efficient Decolorization of the Poly-Azo Dye Sirius Grey by Coriolopsis gallica Laccase-Mediator System: Process Optimization and Toxicity Assessment

The textile industry produces high volumes of colored effluents that require multiple treatments to remove non-adsorbed dyes, which could be recalcitrant due to their complex chemical structure. Most of the studies have dealt with the biodegradation of mono or diazo dyes but rarely with poly-azo dyes. Therefore, the aim of this paper was to study the biodegradation of a four azo-bond dye (Sirius grey) and to optimize its decolorization conditions. Laccase-containing cell-free supernatant from the culture of a newly isolated fungal strain, Coriolopsis gallica strain BS9 was used in the presence of 1-hydroxybenzotriazol (HBT) to optimize the dye decolorization conditions. A Box-Benken design with four factors, namely pH, enzyme concentration, HBT concentration, and dye concentration, was performed to determine optimal conditions for the decolorization of Sirius grey. The optimal conditions were pH 5, 1 U/mL of laccase, 1 mM of HBT, and 50 mg/L of initial dye concentration, ensuring a decolorization yield and rate of 87.56% and 2.95%/min, respectively. The decolorized dye solution showed a decrease in its phytotoxicity (Germination index GI = 80%) compared to the non-treated solution (GI = 29%). This study suggests that the laccase-mediator system could be a promising alternative for dye removal from textile wastewater.

Production, purification, and characterization of p-diphenol oxidase (PDO) enzyme from lignolytic fungal isolate Schizophyllum commune MF-O5

Fungi are producers of lignolytic extracellular enzymes which are used in industries like textile, detergents, biorefineries, and paper pulping. This study assessed for the production, purification, and characterization of novel p-diphenol oxidase (PDO; laccase) enzyme from lignolytic white-rot fungal isolate. Fungi samples collected from different areas of Pakistan were initially screened using guaiacol plate method. The maximum PDO producing fungal isolate was identified on the basis of ITS (internal transcribed spacer sequence of DNA of ribosomal RNA) sequencing. To get optimum enzyme yield, various growth and fermentation conditions were optimized. Later PDO was purified using ammonium sulfate precipitation, size exclusion, and anion exchange chromatography and characterized. It was observed that the maximum PDO producing fungal isolate was Schizophyllum commune (MF-O5). Characterization results showed that the purified PDO was a monomeric protein with a molecular mass of 68 kDa and showed stability at lower temperature (30 °C) for 1 h. The Km and Vmax values of the purified PDO recorded were 2.48 mM and 6.20 U/min. Thermal stability results showed that at 30 °C PDO had 119.17 kJ/K/mol Ea value and 33.64 min half-life. The PDO activity was stimulated by Cu2+ ion at 1.0 mM showing enhanced activity up to 111.04%. Strong inhibition effect was noted for Fe2+ ions at 1 mM showing 12.04% activity. The enzyme showed stability against 10 mM concentration oxidizing reducing agents like DMSO, EDTA, H2O2, NaOCl, and urea and retained more than 75% of relative activity. The characterization of purified PDO enzyme confirmed its tolerance against salt, metal ions, organic solvents, and surfactants indicating its ability to be used in the versatile commercial applications.

The secretome of Agaricus bisporus: Temporal dynamics of plant polysaccharides and lignin degradation

Despite substantial lignocellulose conversion during mycelial growth, previous transcriptome and proteome studies have not yet revealed how secretomes from the edible mushroom Agaricus bisporus develop and whether they modify lignin models in vitro. To clarify these aspects, A. bisporus secretomes collected throughout a 15-day industrial substrate production and from axenic lab-cultures were subjected to proteomics, and tested on polysaccharides and lignin models. Secretomes (day 6-15) comprised A. bisporus endo-acting and substituent-removing glycoside hydrolases, whereas β-xylosidase and glucosidase activities gradually decreased. Laccases appeared from day 6 onwards. From day 10 onwards, many oxidoreductases were found, with numerous multicopper oxidases (MCO), aryl alcohol oxidases (AAO), glyoxal oxidases (GLOX), a manganese peroxidase (MnP), and unspecific peroxygenases (UPO). Secretomes modified dimeric lignin models, thereby catalyzing syringylglycerol-β-guaiacyl ether (SBG) cleavage, guaiacylglycerol-β-guaiacyl ether (GBG) polymerization, and non-phenolic veratrylglycerol-β-guaiacyl ether (VBG) oxidation. We explored A. bisporus secretomes and insights obtained can help to better understand biomass valorization.

Laccase mediated delignification of wasted and non-food agricultural biomass: Recent developments and challenges

Utilization of microbial laccases is considered as the cleaner and target specific biocatalytic mechanism for the recovery of cellulose and hemicelluloses from nonfood and wasted agricultural, lignocellulosic biomass (LCB). The extent of lignin removal by laccase depends on the biochemical composition of biomass and the redox potential (E⁰) of the biocatalyst. Intensive research efforts are going on all over the world for the recognition of appropriate and easily available agricultural lignocellulosic feedstocks to exploit maximally for the production of value-added bioproducts and biofuels. In such circumstances, laccase can play a major role as a leading biocatalyst and potent substitute for chemical based deconstruction of the lignocellulosic materials. The limited commercialization of laccase at an industrial scale has been feasible due to its full working efficiency mostly expressed in the presence of cost intensive redox mediators only. Although, recently there are some reports that came on the mediator free biocatalysis of enzyme but still not considerably explored and neither understood in depth. The present review will address the various research gaps and shortcomings that acted as the big hurdles before the complete exploitation of laccases at an industrial scale. Further, this article also reveals insights on different microbial laccases and their diverse functional environmental conditions that affect the deconstruction process of LCB.

Applications and immobilization strategies of the copper-centred laccase enzyme; a review

Laccase is a multi-copper enzyme widely expressed in fungi, higher plants, and bacteria which facilitates the direct reduction of molecular oxygen to water (without hydrogen peroxide production) accompanied by the oxidation of an electron donor. Laccase has attracted attention in biotechnological applications due to its non-specificity and use of molecular oxygen as secondary substrate. This review discusses different applications of laccase in various sectors of food, paper and pulp, waste water treatment, pharmaceuticals, sensors, and fuel cells. Despite the many advantages of laccase, challenges such as high cost due to its non-reusability, instability in harsh environmental conditions, and proteolysis are often encountered in its application. One of the approaches used to minimize these challenges is immobilization. The various methods used to immobilize laccase and the different supports used are further extensively discussed in this review.

Designed High-Redox Potential Laccases Exhibit High Functional Diversity

White-rot fungi secrete an impressive repertoire of high-redox potential laccases (HRPLs) and peroxidases for efficient oxidation and utilization of lignin. Laccases are attractive enzymes for the chemical industry due to their broad substrate range and low environmental impact. Since expression of functional recombinant HRPLs is challenging, however, iterative-directed evolution protocols have been applied to improve their expression, activity, and stability. We implement a rational, stabilize-and-diversify strategy to two HRPLs that we could not functionally express. First, we use the PROSS stability-design algorithm to allow functional expression in yeast. Second, we use the stabilized enzymes as starting points for FuncLib active-site design to improve their activity and substrate diversity. Four of the FuncLib-designed HRPLs and their PROSS progenitor exhibit substantial diversity in reactivity profiles against high-redox potential substrates, including lignin monomers. Combinations of 3-4 subtle mutations that change the polarity, solvation, and sterics of the substrate-oxidation site result in orders of magnitude changes in reactivity profiles. These stable and versatile HRPLs are a step toward generating an effective lignin-degrading consortium of enzymes that can be secreted from yeast. The stabilize-and-diversify strategy can be applied to other challenging enzyme families to study and expand the utility of natural enzymes.

Bleach enhancement of mixed wood pulp by mixture of thermo-alkali-stable xylanase and mannanase derived through co-culturing of Alkalophilic Bacillus sp. NG-27 and Bacillus nealsonii PN-11

The Application of a combination of enzymes is the best alternative to reduce the use of chemicals in the paper industry. Bacillus sp. NG-27 and Bacillus nealsonii PN-11 are known to produce thermoalkali stable xylanse (X) and mannanase (M) respectively having potential for pulp biobleaching. The Present study, reports the production of a mixture of X + M by co-culturing of strains in SSF and standardizing its application for pulp biobleaching. Production of enzymes by co-cultivation in SSF was optimized by statistical methods. Substantial increase in the yield of enzymes; 3.61 fold of xylanase and 37.71 fold of mannanase was achieved. Application of enzyme cocktail for pulp biobleaching resulted in a 45.64% reduction of kappa number with 55 IU g^-1odp of enzyme dose (xylanase:mannanase; 3:1) at pH 8.0 in 1h at 65 °C along with significant increase in brightness (11%) and whiteness (75%). The Same quality of paper as made up from chemical treated pulp can be made from enzyme-treated pulp with 30% less use of chlorine. Structural analysis of enzyme-treated pulp showed dissolution of hemicellulose as indicated by pores, cracks and increased roughness all over the surface. Cocktail of X + M produced economically in a single fermentation having all the requisite characteristics for pulp biobleaching is a highly suitable candidate for application in the pulp and paper industry.

Rhus vernicifera Laccase Immobilization on Magnetic Nanoparticles to Improve Stability and Its Potential Application in Bisphenol A Degradation

In the present study, Rhus vernicifera laccase (RvLac) was immobilized through covalent methods on the magnetic nanoparticles. Fe₂O₃ and Fe₃O₄ nanoparticles activated by 3-aminopropyltriethoxysilane followed with glutaraldehyde showed maximum immobilization yields and relative activity up to 81.4 and 84.3% at optimum incubation and pH of 18 h and 5.8, respectively. The maximum RvLac loading of 156 mg/g of support was recorded on Fe₂O₃ nanoparticles. A higher optimum pH and temperature of 4.0 and 45 °C were noted for immobilized enzyme compared to values of 3.5 and 40 °C for free form, respectively. Immobilized RvLac exhibited better relative activity profiles at various pH and temperature ranges. The immobilized enzyme showed up to 16-fold improvement in the thermal stability, when incubated at 60 °C, and retained up to 82.9% of residual activity after ten cycles of reuses. Immobilized RvLac exhibited up to 1.9-fold higher bisphenol A degradation efficiency potential over free enzyme. Previous reports have demonstrated the immobilization of RvLac on non-magnetic supports. This study has demonstrated that immobilization of RvLac on magnetic nanoparticles is very efficient especially for achieving high loading, better pH and temperature profiles, and thermal- and solvents-stability, high reusability, and higher degradation of bisphenol A.

Enhancing the decolorization activity of Bacillus pumilus W3 CotA-laccase to Reactive Black 5 by site-saturation mutagenesis

Reactive Black 5 (RB5) is a typical refractory azo dye. Widespread utilization of RB5 has caused a variety of environmental and health problems. The enzymatic degradation of RB5 can be a promising solution due to its superiority as an eco-friendly and cost-competitive process. Bacterial CotA-laccase shows great application prospect to eliminate hazardous dyes from wastewater. However, efficient decolorization of RB5 CotA-laccase generally requires the participation of costly, toxic mediators. In the present study, we modified the amino acids Thr415 and Thr418 near the type 1 copper site and the amino acid Gln442 at the entrance of the substrate-binding pocket of Bacillus pumilus W3 CotA-laccase to boost its RB5 decolorization activity based on molecular docking analysis and site-saturation mutagenesis. Through the strategies, two double site mutants T415D/Q442A and T418K/Q442A obtained demonstrated 43.94 and 52.64% RB5 decolorization rates in the absence of a mediator at pH 10.0, respectively, which were about 3.70- and 4.43-fold higher compared with the wild-type CotA-laccase. Unexpectedly, the catalytic efficiency of the T418K/Q442A to ABTS was enhanced by 5.33-fold compared with the wild-type CotA-laccase. The mechanisms of conferring enhanced activity to the mutants were proposed by structural analysis. In summary, the mutants T415D/Q442A and T418K/Q442A have good application potentials for the biodegradation of RB5. KEY POINTS: • Three amino acids of CotA-laccase were manipulated by site-saturation mutagenesis. • Decolorization rate of two mutants to RB5 was enhanced 3.70- and 4.43-fold, respectively. • The mechanisms of awarding enhanced activity to the mutants were supposed.

Laccase activity of the ascomycete fungus Nectriella pironii and innovative strategies for its production on leaf litter of an urban park

A laccase-producing ascomycete fungus was isolated from soil collected around the premises of a textile dye factory and identified as Nectriella pironii. Efficient laccase production was achieved via the synergistic action of 1 mM copper sulfate and ferulic acid. Extracts of rapeseed oil cake, grass hay, and leaf litter collected in a pocket urban park were used for enzyme production. The highest laccase activity (3,330 U/L) was observed in the culture grown on the leaf litter extract. This is the first report on biosynthesis of laccase by N. pironii. This is also the first study on utilization of naturally fallen park leaves as a substrate for fungal laccase production. The extracellular enzyme possessing laccase activity was purified to homogeneity by ion-exchange and gel filtration chromatographic techniques. The amino acid sequence of the protein revealed highest similarity to the laccase enzyme produced by Stachybotrys chartarum—and considerable homology to those produced by other fungal species. The purified laccase possessed a molecular mass of 50 kDa. The enzyme had an optimum pH of 2.0 or 6.0 and retained more than 50% of residual activity after 3 hours of incubation at pH 3.0–10.6 or 4.0–9.0 when 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid or 2,6-dimethoxyphenol, respectively, were used. Dithiothreitol, β-mercaptoethanol, and sodium azide at 1 mM concentration strongly inhibited the laccase activity, while in the presence of 50 mM urea, the enzyme was found to retain 25% of its activity. The laccase was able to decolorize more than 80% of Indigo Carmine, Remazol Brilliant Blue R, Reactive Orange 16, and Acid Red 27 dyes within 1 h. The possibility of leaf litter use for the production of the laccase enzyme from N. pironii (IM 6443), exhibiting high pH stability and degradative potential, makes it a promising tool for use in different environmental and industrial operations.

Discovery of hyperstable carbohydrate-active enzymes through metagenomics of extreme environments

The enzymes from hyperthermophilic microorganisms populating volcanic sites represent interesting cases of protein adaptation and biotransformations under conditions where conventional enzymes quickly denature. The difficulties in cultivating extremophiles severely limit access to this class of biocatalysts. To circumvent this problem, we embarked on the exploration of the biodiversity of the solfatara Pisciarelli, Agnano (Naples, Italy), to discover hyperthermophilic carbohydrate-active enzymes (CAZymes) and to characterize the entire set of such enzymes in this environment (CAZome). Here, we report the results of the metagenomic analysis of two mud/water pools that greatly differ in both temperature and pH (T = 85 °C and pH 5.5; T = 92 °C and pH 1.5, for Pool1 and Pool2, respectively). DNA deep sequencing and following in silico analysis led to 14 934 and 17 652 complete ORFs in Pool1 and Pool2, respectively. They exclusively belonged to archaeal cells and viruses with great genera variance within the phylum Crenarchaeota, which reflected the difference in temperature and pH of the two Pools. Surprisingly, 30% and 62% of all of the reads obtained from Pool1 and 2, respectively, had no match in nucleotide databanks. Genes associated with carbohydrate metabolism were 15% and 16% of the total in the two Pools, with 278 and 308 putative CAZymes in Pool1 and 2, corresponding to ~ 2.0% of all ORFs. Biochemical characterization of two CAZymes of a previously unknown archaeon revealed a novel subfamily GH5_19 β-mannanase/β-1,3-glucanase whose hemicellulose specificity correlates with the vegetation surrounding the sampling site, and a novel NAD⁺ -dependent GH109 with a previously unreported β-N-acetylglucosaminide/β-glucoside specificity. DATABASES: The sequencing reads are available in the NCBI Sequence Read Archive (SRA) database under the accession numbers SRR7545549 (Pool1) and SRR7545550 (Pool2). The sequences of GH5_Pool2 and GH109_Pool2 are available in GenBank database under the accession numbers MK869723 and MK86972, respectively. The environmental data relative to Pool1 and Pool2 (NCBI BioProject PRJNA481947) are available in the Biosamples database under the accession numbers SAMN09692669 (Pool1) and SAMN09692670 (Pool2).

Enhanced production, one-step affinity purification, and characterization of laccase from solid-state culture of Lentinus tigrinus and delignification of pistachio shell by free and immobilized enzyme

Laccase mediated bio-delignification has shown promising results for the removal of lignin from bio-wastes and for providing a sustainable future for using of lignocellulosic materials in different industries. This study reports an extracellular laccase from Lentinus tigrinus with delignification capability. The production of laccase was enhanced through a solid-state fermentation on the pistachio shell bio-waste to 172.0 U mg^-1 (8.2-fold) by one-factor-at-a-time optimizing of fermentation conditions. Laccase was purified using a new synthetic affinity resin yielding a specific activity of 543.6 U mg^-1 and a 23.9-fold purification. The purified laccase was then immobilized covalently on the large pore magnetic SBA-15. Compared to free enzyme, immobilized enzyme maintained more stable at pH 2.0-11.0 and 25-55 °C, and against organic solvents, surfactants, metal ions, and inhibitors. The activity of both forms of the enzyme was increased with Cu²⁺, Ca⁺², cetyltrimethylammonium bromide, and ethyl acetate. A 0.72 V redox potential caused enzyme specificity to various substrates. 80% of lignin content of the bio-waste was removed by 50 U mL^-1 of immobilized enzyme after 8 h fermentation and delignification efficiency was greatly increased by applying higher enzyme dosages, surfactants, and organic solvents. In addition, residual activity was more than 50% after 20 cycles of delignification. The results of delignification were confirmed by GC-MS, SEM, and composition analysis of pistachio shells. This study illustrated the notable promise of the enzyme for biotechnological and environmental applications.

Establishing the oxidative tolerance of Thermomyces lanuginosus xylanase

AIMS

This work aims to determine the tolerance of xylanase towards enzyme-generated oxidative conditions, such as those produced by the peroxidase or laccase mediator systems (LMS).

METHODS AND RESULTS

The activity of Thermomyces lanuginosus xylanase was measured after incubation with lignin peroxidase, manganese peroxidase or laccase plus various mediators. The laccase system, using mediators such as 1-hydroxybenzotriazole and violuric acid, resulted in complete loss of xylanase activity, accompanied by an increase in the solution potential. However, an increase in solution potential alone was not sufficient to inactivate xylanase, nor was loss of xylanase activity always accompanied by a significant increase in solution potential, as observed with N-hydroxyphthalimide as the mediator. Neither lignin peroxidase nor manganese peroxidase impacted xylanase activity; only extended treatment with elevated hydrogen peroxide concentration promoted modest xylanase activity loss. The mechanism of inactivation as determined by the tryptophan-modifying reagent N-bromosuccinimide (NBS) indicated that oxidation of just one of the eight tryptophan residues of T. lanuginosus xylanase would be sufficient to result in complete loss of xylanase activity, since xylanase is completely inactivated at 1 : 1 molar ratio of NBS to xylanase.

CONCLUSIONS

While showing tolerance to peroxidase-based enzyme systems, T. lanuginosus xylanase is readily inactivated in the presence of the LMS. Based upon treatment with NBS as the oxidant, inactivation can be attributed to modification of a single tryptophan residue.

SIGNIFICANCE AND IMPACT OF THE STUDY

The simultaneous application of mixed hydrolytic and oxidative enzyme systems is of importance to biomass processing industries. Understanding the tolerance of xylanase to oxidative conditions will facilitate the design of reaction conditions or enzyme variants to maximize the impact of mixed enzyme systems.

Integration of biological pre-treatment methods for increased energy recovery from paper and pulp biosludge

The paper and pulp industry (PPI) produces high quantities of solid and liquid discharge and is regarded as the most polluting industry in the world causing adverse effects to environments and human beings. Hence changes in the way PPI sludge and waste materials are treated is urgently required. Nearly, 10 million tons of waste is generated per year, however PPI waste is enriched with many organic chemicalscontaining a high percentage of lignin, cellulose, and hemicellulose which can be used as valuable raw materials for the production of bioenergy and value-added chemicals. Pretreatment of complex lignocellulosic materials of PPI waste is difficult because of the cellulose crystallinity and lignin barrier. At present most of this waste is recycled in a conventional treatment approach through biological and chemical processes, incurring high cost and low returns. Henceefficient pretreatment techniques are required by which complete conversion of PPI waste is possible. Therefore, the present chapter provides the scope of integration of pretreatment methods through which bioenergy recovery is possible during the PPI waste treatment. Detailed information is presented on the various pre-treatment techniques (chemical, mechanical, enzymatic and biological) in order to increase the efficiency of PPI waste treatment and energy recovery from PPI waste. Along with acid and alkali based efficient chemical treatment process, physical methods (i.e. shearing, high-pressure homogenization, etc.), biochemical techniques (whole cell-based and enzyme-based) and finally biological techniques (e.g. aerobic and anaerobic treatment) are discussed. During each of the treatment processes, scope of energy recovery and bottlenecks of the processes were elaborated. The review thus provides systemic insight into developing efficient pretreatment processes which could increase carbon recovery and treatment efficiency of PPI waste.

Directed Evolution of a Bacterial Laccase (CueO) for Enzymatic Biofuel Cells

Escherichia coli's copper efflux oxidase (CueO) has rarely been employed in the cathodic compartment of enzymatic biofuel cells (EBFCs) due to its low redox potential (0.36 V vs. Ag/AgCl, pH 5.5) towards O₂ reduction. Herein, directed evolution of CueO towards a more positive onset potential was performed in an electrochemical screening system. An improved CueO variant (D439T/L502K) was obtained with a significantly increased onset potential (0.54 V), comparable to that of high-redox-potential fungal laccases. Upon coupling with an anodic compartment, the EBFC exhibited an open-circuit voltage (V_oc ) of 0.56 V. Directed enzyme evolution by tailoring enzymes to application conditions in EBFCs has been validated and might, in combination with molecular understanding, enable future breakthroughs in EBFC performance.

Directed Evolution of a Homodimeric Laccase from Cerrena unicolor BBP6 by Random Mutagenesis and In Vivo Assembly

Laccases have great potential for industrial applications due to their green catalytic properties and broad substrate specificities, and various studies have attempted to improve the catalytic performance of these enzymes. Here, to the best of our knowledge, we firstly report the directed evolution of a homodimeric laccase from Cerrena unicolor BBP6 fused with α-factor prepro-leader that was engineered through random mutagenesis followed by in vivo assembly in Saccharomyces cerevisiae. Three evolved fusion variants selected from ~3500 clones presented 31- to 37-fold increases in total laccase activity, with better thermostability and broader pH profiles. The evolved α-factor prepro-leader enhanced laccase expression levels by up to 2.4-fold. Protein model analysis of these variants reveals that the beneficial mutations have influences on protein pKa shift, subunit interaction, substrate entrance, and C-terminal function.

Enzymatic Processes to Unlock the Lignin Value

Main hurdles of lignin valorization are its diverse chemical composition, recalcitrance, and poor solubility due to high-molecular weight and branched structure. Controlled fragmentation of lignin could lead to its use in higher value products such as binders, coatings, fillers, etc. Oxidative enzymes (i.e., laccases and peroxidases) have long been proposed as a potentially promising tool in lignin depolymerization. However, their application was limited to ambient pH, where lignin is poorly soluble in water. A Finnish biotechnology company, MetGen Oy, that designs and supplies industrial enzymes, has developed and brought to market several lignin oxidizing enzymes, including an extremely alkaline lignin oxidase MetZyme^® LIGNO™, a genetically engineered laccase of bacterial origin. This enzyme can function at pH values as high as 10–11 and at elevated temperatures, addressing lignin at its soluble state. In this article, main characteristics of this enzyme as well as its action on bulk lignin coming from an industrial process are demonstrated. Lignin modification by MetZyme^® LIGNO™ was characterized by size exclusion chromatography, UV spectroscopy, and dynamic light scattering for monitoring particle size of solubilized lignin. Under highly alkaline conditions, laccase treatment not only decreased molecular weight of lignin but also increased its solubility in water and altered its dispersion properties. Importantly, organic solvent-free soluble lignin fragmentation allowed for robust industrially relevant membrane separation technologies to be applicable for product fractionation. These enzyme-based solutions open new opportunities for biorefinery lignin valorization thus paving the way for economically viable biorefinery business.

Emergent contaminants: Endocrine disruptors and their laccase-assisted degradation - A review

Herein, an effort has been made to highlight the trends of the state-of-the-art of laccase-assisted degradation of emerging contaminants at large and endocrine disruptors in particular. Since first described in the 19th century, laccase has received particular interest for inter- and multidisciplinary investigations due to its uniqueness and remarkable biotechnological applicability. There has always been a paramount concern over the widespread occurrences of various pollutant types, around the globe. Therefore, pollution free processes are gaining ground all over the world. With ever increasing scientific knowledge, socioeconomic awareness, human health-related issues and ecological apprehensions, people are more concerned about the widespread environmental pollutants. In this context, the occurrences of newly identified pollutants so-called "emerging contaminants - ECs" in our main water bodies is of continued and burning concern worldwide. Undoubtedly, various efforts have already been made to tackle this challenging ECs concern though using different approaches including physical and chemical, however, each has considerable limitations. In this review, we present information on how laccase-assisted approach can change this limited tendency of physical and chemical based approaches. A special focus has been given to the laccase-assisted systems including pristine laccase, laccase-mediator catalyzed system and immobilized-laccase catalyzed system that promotes the endocrine disruptors removal. Towards the end, a list of outstanding questions and research gaps are given that can pave the way for future studies.

Immobilized Cerrena sp. laccase: preparation, thermal inactivation, and operational stability in malachite green decolorization

Laccases are polyphenol oxidases with widespread applications in various industries. In the present study, the laccase from Cerrena sp. HYB07 was immobilized with four methods, namely entrapment in alginate, covalently binding to chitosan as well as formation of cross-linked enzyme aggregates (CLEAs) and magnetic CLEAs (M-CLEAs). The activity recovery rates of the immobilized laccases ranged from 29% to 68%. Immobilization elevated the reaction temperature optimum and reduced substrate specificity, but not necessarily the turnover rate. pH stability of immobilized laccases was improved compared with that of the free laccase, especially at acidic pH values. Thermal inactivation of all laccases followed a simple first-order exponential decay model, and immobilized laccases displayed higher thermostability, as manifested by lower thermal inactivation rate constants and longer enzyme half-life time. Operational stability of the immobilized laccase was demonstrated by decolorization of the triphenylmethane dye malachite green (MG) at 60 °C. MG decolorization with free laccase was accompanied by a shift of the absorption peak and accumulation of a stable, colored intermediate tetradesmethyl MG, probably due to lower thermostability of the free laccase and premature termination of the degradation pathway. In contrast, complete decolorization of MG was achieved with laccase CLEAs at 60 °C.

Laccases: Production, Expression Regulation, and Applications in Pharmaceutical Biodegradation

Laccases are a family of copper-containing oxidases with important applications in bioremediation and other various industrial and biotechnological areas. There have been over two dozen reviews on laccases since 2010 covering various aspects of this group of versatile enzymes, from their occurrence, biochemical properties, and expression to immobilization and applications. This review is not intended to be all-encompassing; instead, we highlighted some of the latest developments in basic and applied laccase research with an emphasis on laccase-mediated bioremediation of pharmaceuticals, especially antibiotics. Pharmaceuticals are a broad class of emerging organic contaminants that are recalcitrant and prevalent. The recent surge in the relevant literature justifies a short review on the topic. Since low laccase yields in natural and genetically modified hosts constitute a bottleneck to industrial-scale applications, we also accentuated a genus of laccase-producing white-rot fungi, Cerrena, and included a discussion with regards to regulation of laccase expression.

High Laccase Expression by Trametes versicolor in a Simulated Textile Effluent with Different Carbon Sources and PHs

Textile effluents are highly polluting and have variable and complex compositions. They can be extremely complex, with high salt concentrations and alkaline pHs. A fixed-bed bioreactor was used in the present study to simulate a textile effluent treatment, where the white-rot fungus, Trametes versicolor, efficiently decolourised the azo dye Reactive Black 5 over 28 days. This occurred under high alkaline conditions, which is unusual, but advantageous, for successful decolourisation processes. Active dye decolourisation was maintained by operation in continuous culture. Colour was eliminated during the course of operation and maximum laccase (Lcc) activity (80.2 U∙L⁻¹) was detected after glycerol addition to the bioreactor. Lcc2 gene expression was evaluated with different carbon sources and pH values based on reverse transcriptase-PCR (polymerase chain reaction). Glycerol was shown to promote the highest lcc2 expression at pH 5.5, followed by sucrose and then glucose. The highest levels of expression occurred between three and four days, which corroborate the maximum Lcc activity observed for sucrose and glycerol on the bioreactor. These results give new insights into the use of T. versicolor in textile dye wastewater treatment with high pHs.

Effect of Direct-Current Electric Field on Enzymatic Activity and the Concentration of Laccase

This work investigates the effect of direct-current electric field on the extracellular enzymatic activity, concentration and other experimental parameters of laccase from Trametes versicolor. The results showed that laccase could significantly contribute to the change of pH at the end of graphite electrode. In addition, it increased the electrical conductivity of the water. In the experiment, the optimum pH and catalytic pH range for laccase activity were 3.0 and pH 2.5-4.0. The application of 6 V direct current showed significant effects on the laccase enzyme activity. The activity of laccase was enhanced in the anodic region, but at the same time was strongly inhibited at the cathode. The electric charge characteristics of laccase were changed when exposed to electric field, and some laccases molecules moved to the anode, which produced a slight migration phenomenon. This study is the basis of combination of laccase and electrical technology, at the same time, providing a new direction of enhancing laccase activity. Compared to immobilization, using electric field is simple, no chemical additives, and great potential.